Learn Hardcore Greek: Χτες ο μέσος όρος στην εξέταση ελληνικών ήταν καλός. - Yesterday the average in the Greek exam was good.

Χτες (yesterday) is an adverb of time. In Greek, adverbs of time are often placed at the beginning of the sentence for a neutral, natural-sounding word order:

Χτες ο μέσος όρος στην εξέταση ελληνικών ήταν καλός.
Yesterday the average in the Greek exam was good.

You can move χτες without changing the basic meaning, but the focus and naturalness change a bit:

Ο μέσος όρος στην εξέταση ελληνικών χτες ήταν καλός.
Also possible, but sounds a bit heavier and puts extra focus on χτες.
Ο μέσος όρος χτες στην εξέταση ελληνικών ήταν καλός.
Also grammatically correct, but the original sentence is smoother.

So: yes, χτες can move, but sentence-initial position is the most neutral and common here.

What exactly is ο μέσος όρος grammatically, and why do we use ο here?

Ο μέσος όρος means the average.

Grammatically:

ο = definite article, masculine, singular, nominative
μέσος = adjective (middle / average), masculine, singular, nominative
όρος = noun (term / value / limit, here: average), masculine, singular, nominative

In Greek, adjectives agree with the noun in:

gender (masculine here),
number (singular),
case (nominative, because it’s the subject).

So:

ο μέσος όρος = the average (subject of the sentence)

You cannot drop ο in normal Greek; in this context you must say ο μέσος όρος, not just μέσος όρος.

Why is it ήταν καλός and not ήταν καλό or something else?

Ήταν is the past tense of είναι (to be), and καλός is an adjective meaning good.

The key point: καλός must agree with ο μέσος όρος:

ο μέσος όρος → masculine, singular, nominative
So the adjective must be: καλός (masculine, singular, nominative)

If the subject were feminine or neuter, the adjective would change:

Η εξέταση ήταν καλή.
The exam was good. (feminine: η εξέταση, καλή)
Το αποτέλεσμα ήταν καλό.
The result was good. (neuter: το αποτέλεσμα, καλό)

Here, because μέσος όρος is masculine, we must say ήταν καλός.

What is ήταν exactly? Is it the same as ήτανε, and how is it different from είναι?

είναι = is/are (present tense of είμαι = to be)
ήταν = was/were (past tense, 3rd person singular & plural)

So:

Ο μέσος όρος είναι καλός.
The average is good. (now / generally)
Χτες ο μέσος όρος ήταν καλός.
Yesterday the average was good. (past)

ήτανε is just a more colloquial/alternative form of ήταν. In most contexts:

ήταν and ήτανε mean the same thing.
ήταν is more common and more neutral in writing.

What does στην mean here, and why not σε την or στη?

στην is a contraction of the preposition σε + the article την:

σε = in / at / on
την = feminine, singular, accusative article (the)
σε + την εξέταση → στην εξέταση

That’s why it means in the exam.

You’ll often see these contractions in Greek:

σε + τον → στον (e.g. στον δρόμο – in the street)
σε + την → στην (e.g. στην εξέταση – in the exam)
σε + το → στο (e.g. στο σπίτι – at home)

στη (without ν) also exists: it’s the same word, just without the final -ν, which may be dropped before some consonants in casual speech. Before εξέταση (starting with a vowel), στην εξέταση is the standard form.

Why is it στην εξέταση ελληνικών and not something like στην ελληνική εξέταση?

Both are possible, but they’re not exactly the same:

στην εξέταση ελληνικών
Literally: in the exam of Greek (language)
- εξέταση = exam/test
- ελληνικών = of Greek (language), genitive plural
  This sounds like the exam in the subject “Greek” (like English exam, math exam, etc.). Very natural in a school/university context.
στην ελληνική εξέταση
Literally: in the Greek exam
- ελληνική = Greek (adjective), feminine, singular
- εξέταση = exam
  This can also work, but it can be understood more as the exam which is Greek in some way (e.g. a Greek-style exam, an exam in Greece, etc.). In practice, people more often say στην εξέταση ελληνικών for “the Greek exam (language subject)”.

So the original phrase is the more standard way to say in the Greek (language) exam.

Why is ελληνικών in the plural and in that -ων form? What is it grammatically?

ελληνικών here is:

Genitive
Plural
Of τα ελληνικά (the Greek language)

In Greek:

η ελληνική γλώσσα = the Greek language (singular)
τα ελληνικά = Greek (as a language, literally “the Greek things/words”); this is very common in everyday speech.

The genitive plural of τα ελληνικά is:

των ελληνικών = of (the) Greek (language)

In στην εξέταση ελληνικών, the των is dropped, but the genitive plural form ελληνικών is kept. This is a common pattern after a noun to show the subject/field/content of something:

εξέταση ελληνικών = exam of Greek
μάθημα μαθηματικών = math class (class of mathematics)
τεστ ιστορίας = history test (test of history)

So ελληνικών is genitive plural describing the content/subject of the exam.

Why is ελληνικών written with a lowercase ε, but in English “Greek” is capitalized?

Capitalization rules are different in Greek and English.

In Greek:

Nationality adjectives and language names are normally written with a lowercase first letter:
- ελληνικά (Greek)
- αγγλικά (English)
- γαλλικά (French)

In English:

Language and nationality words are capitalized:
- Greek, English, French

So:

στην εξέταση ελληνικών → “in the Greek exam”
The ε in ελληνικών is lowercase in correct Greek spelling, even though English would write Greek with a capital G.

Where is the article for “exam” in στην εξέταση ελληνικών? Why don’t I see την separately?

The article την is actually hidden inside στην:

στην = σε + την
εξέταση = exam (feminine)
στην εξέταση = in the exam

So the full structure is:

στην (σε + την) εξέταση ελληνικών
Literally: in the exam of Greek

You don’t see την as a separate word because it’s fused with σε. This is standard in Greek—it’s not optional in normal usage.

Is Χτες the only correct form, or can I also write Χθες, Εχτές, Εχθές?

You will see several forms, and they are all understood:

Χτες
Χθες
Εχτές
Εχθές

Differences:

The χθ- / χτ- / εχθ- / εχτ- variation is mostly historical/phonetic; in modern spoken Greek the θ is often not pronounced or is weakened.
The ε- at the beginning (εχτές) is just an extra vowel that appears in speech.

In practice:

Χτες and Χθες are very common in writing.
Εχτές / Εχθές sound a bit more colloquial or dialectal, but are still used.

For learning purposes, you can safely use Χτες; it’s short, common, and perfectly correct.